'American Idol' crowns final winner

The farewell season finale of American Idol opened with a message from President Barack Obama, who used the opportunity to talk about the importance of voting. "I believe it should be almost as easy as voting for American Idol, and we're working on that."

The first performance of the night came from the farewell season's Top 10, who sang "One Voice" by Barry Manilow. Over the course of the song, they were gradually joined by past Idol winners and contestants, including Kellie Pickler, Taylor Hicks, Katharine McPhee, Constantine Maroulis and others.

Idol host Ryan Seacrest walked out onto the stage one last time, and shortly after he was joined by Brian Dunkleman, who co-hosted the first season of the show alongside Seacrest but didn't stick around. "Looks like after tonight, you're gonna be out of a job," Dunkelman joked to Seacrest.

Seacrest then brought out season 15's judges: Harry Connick Jr., Jennifer Lopez and Keith Urban, who walked out onto the Dolby Theatre stage holding hands.

SEE ALSO: 'American Idol' Farewell Season Finalists Sing One Last Time for America's Vote

Finalists La'Porsha Renae and Trent Harmon then reprised Kelly Clarkson and Justin Guarini's duet of "It Takes Two" by Marvin Gaye. Clarkson and Guarini performed the duet during the show's first season finale. Renae and Harmon sounded great together, their voices blending very well.

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It was a night full of throwbacks and tributes. The first medley kicked off with Kara Dioguardi, who was a judge during seasons 8 and 9. She sang Pink's "Sober" with Jordin Sparks and Tamyra Gray. Season 11's Colton Dixon followed by singing his song "Through All Of It." Sparks performed her hit song "No Air," with Justin Guarini doing the Chris Brown part. Season 2's Kimberley Locke sang "8th World Wonder," which led into Guarini, Sparks and Gray singing "Girl Crush" by Little Big Town. Season 8's Allison Iraheta also joined, singing Meghan Trainor's "No," which turned into a mashup with Larry Pratt's iconic "Pants On The Ground" audition. Season 10's Pia Toscano, whose early elimination shocked judges Randy Jackson, Steven Tyler and Lopez at the time, returned to sing "All By Myself" by Celine Dion.

As Seacrest explained, Clarkson was unable to be at the live finale event because she is about to have her baby. But she filmed a medley of her own hits ahead of time. The performance included parts of "Miss Independent," "Breakaway," "Since U Been Gone," "Because Of You," "Walk Away," "My Life Would Suck Without You", "Already Gone," "Stronger" and "Invincible," highlighting just how many hit singles she has had over the years. It ended with an acapella performance of "A Moment Like This."

SEE ALSO: 'American Idol' Finale: Nigel Lythgoe Reveals White Guys With Guitars Medley, Bowie Tribute and More

Idol's rockers then came to the stage for a rock 'n' roll medley. The rocker performance featured season 4's runner-up Bo Bice, Maroulis, season 13's winner Caleb Johnson, season 10's James Durbin and season 5's Chris Daughtry.

Urban then took the stage with the special guest they've been teasing all week: Season four winner Carrie Underwood. They sang "Stop Draggin' My Heart Around," and it was one of the best performances of the night. Fittingly, they were followed by a country medley featuring Diana Degarmo, Skylar Laine, Kree Harrison, Bucky Covington, Ace Young, Lauren Alaina, Kelly Pickler, Scotty McCreery and Maroulis.

Connick then paid tribute to the Ellis Marsalis Center for Music in New Orleans, singing a duet of "What A Wonderful World" with one of the center's young students, Marley Fletcher.

Their moving performance was followed by quite the lineup of power ballads. Casey James and McPhee sang "Need You Now" by Lady Antebellum. Season 7's Carly Smithson sang "Here You Come Again." Season two runner-up Clay Aiken sang John Denver's "Annie's Song." Season two winner Ruben Studdard teamed up with Amber Holcomb to sing "Here, There And Everywhere" by The Beatles, and season 11's Jessica Sanchez hit some impressive high notes on "The Prayer."

Lopez then debuted her new single "Ain't Your Mama," which she teased on Twitter earlier in the week. She also performed "Let's Get Loud," giving a little taste of what her Las Vegas residency performances have been like.

LaToya London, Fantasia Barrino and Jennifer Hudson reunited for a stunning acapella performance of "Bridge Over Troubled Water." Hudson then performed her song from Empire, "Remember The Music." Brandon Rogers, Clark Beckham, Danny Gokey, George Huff and Elliott Yamin also returned to the Idol istage. Fantasia did a solo performance of her single "Ugly," and then London came back out to sing with Taylor Hicks. Joshua Ledet from season 11 then sang "It's A Man's Man's Man's World." Season 12 winner Candice Glover and season 6's Melinda Doolittle joined the stage to close out the medley.

Past winners Phillip Phillips, Lee Dewyze, David Cook, Nick Fradiani and Kris Allen then paid tribute to David Bowie.

After a video montage of some of the best Simon Cowell, Paula Abdul and Randy Jackson moments from the early years, original judges Abdul and Jackson came to the stage without Cowell. Abdul and Jackson presented Seacrest with a giant American Idol logo to honor the fact that he is the only person who has stayed around all 15 seasons. Cowell then surprised the others by sneaking up behind them. He jokingly asked Abdul to apologize for how mean she had been to contestants during their run. "Thank you, America, for inviting us into your homes," Cowell said. "Without the fans, there would be no show."

SEE ALSO: American Idol' Special Looks Back at Past 15 Years

William Hung also surprised the audience by reprising a performance of "She Bangs."

After a look back at all the sponsored Ford videos through the seasons, Underwood returned to the stage to perform "Something In The Water," which she will be singing on her upcoming tour.

It was finally time for Seacrest to crown the winner. After the nationwide vote, Trent Harmon became the 15th and final American Idol. He performed the single he debuted last night, "Falling." This time, he performed it as the farewell season champion.

"Goodnight, America...for now," Seacrest said, signing off.